linux-ext4 - Re: [PATCH 19/31] fuse: query filesystem geometry when using iomap

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Message-ID: <20260206021757.GH7686@frogsfrogsfrogs>
Date: Thu, 5 Feb 2026 18:17:57 -0800
From: "Darrick J. Wong" <djwong@...nel.org>
To: Chris Mason <clm@...a.com>
Cc: miklos@...redi.hu, joannelkoong@...il.com, bernd@...ernd.com,
	neal@...pa.dev, linux-ext4@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 19/31] fuse: query filesystem geometry when using iomap

On Thu, Feb 05, 2026 at 11:07:55AM -0800, Chris Mason wrote:
> "Darrick J. Wong" <djwong@...nel.org> wrote:
> > From: Darrick J. Wong <djwong@...nel.org>
> > 
> > Add a new upcall to the fuse server so that the kernel can request
> > filesystem geometry bits when iomap mode is in use.
> > 
> 
> Hi everyone,
> 
> I'm trying out my AI review prompts on a few more trees, and I ran it
> on the fuse-iomap-cache branch:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux.git/log/?h=fuse-iomap-cache
> 
> This was the latest email lore could find, apologies if I grabbed the wrong one.
> 
> Hopefully the review below is useful, please let me know if there are places it
> has gone wildly wrong.  I kept the header to make it obvious exactly which
> commit I'd run through:
> 
> commit d529d13100b2e00c33a507c371b6c1b19f2206ae
> Author: Darrick J. Wong <djwong@...nel.org>
> 
> fuse_trace: query filesystem geometry when using iomap
> 
> This commit adds tracepoints for the FUSE_IOMAP_CONFIG operation,
> enabling tracing of filesystem geometry queries when using iomap with
> FUSE. The tracepoint captures configuration flags, block size, link
> limits, time granularity and ranges, maximum file size, and UUID length.
> 
> > diff --git a/fs/fuse/fuse_iomap.c b/fs/fuse/fuse_iomap.c
> > --- a/fs/fuse/fuse_iomap.c
> > +++ b/fs/fuse/fuse_iomap.c
> > @@ -743,6 +743,8 @@ static int fuse_iomap_process_config(struct fuse_mount *fm, int error,
> >  		return error;
> >  	}
> >
> > +	trace_fuse_iomap_config(fm, outarg);
> > +
> >  	if (outarg->flags & ~FUSE_IOMAP_CONFIG_ALL)
> >  		return -EINVAL;
> >
> 
> [ ... ]
> 
> > diff --git a/fs/fuse/fuse_trace.h b/fs/fuse/fuse_trace.h
> > --- a/fs/fuse/fuse_trace.h
> > +++ b/fs/fuse/fuse_trace.h
> 
> [ ... ]
> 
> > @@ -1005,6 +1014,45 @@
> >  		  __entry->mode,
> >  		  __entry->newsize)
> >  );
> > +
> > +TRACE_EVENT(fuse_iomap_config,
> > +	TP_PROTO(const struct fuse_mount *fm,
> > +		 const struct fuse_iomap_config_out *outarg),
> > +	TP_ARGS(fm, outarg),
> > +
> > +	TP_STRUCT__entry(
> > +		__field(dev_t,			connection)
> > +
> > +		__field(uint32_t,		flags)
> > +		__field(uint32_t,		blocksize)
> > +		__field(uint32_t,		max_links)
> > +		__field(uint32_t,		time_gran)
> > +
> > +		__field(int64_t,		time_min)
> > +		__field(int64_t,		time_max)
> > +		__field(int64_t,		maxbytes)
> > +		__field(uint8_t,		uuid_len)
> > +	),
> > +
> > +	TP_fast_assign(
> > +		__entry->connection	=	fm->fc->dev;
> > +		__entry->flags		=	outarg->flags;
>                                                            ^^^^
> 
> Does this truncate the upper 32 bits of the flags field? The flags field
> in struct fuse_iomap_config_out (include/uapi/linux/fuse.h:1471) is
> defined as uint64_t:
> 
> struct fuse_iomap_config_out {
> 	uint64_t flags;		/* FUSE_IOMAP_CONFIG_* */
> 	...
> }
> 
> But the trace event declares it as uint32_t in TP_STRUCT__entry. While
> current flag definitions (FUSE_IOMAP_CONFIG_SID through
> FUSE_IOMAP_CONFIG_MAXBYTES) only use bits 0-5, using uint32_t will lose
> the upper 32 bits if future flags use them.
> 
> Should the trace event field be declared as uint64_t to match the
> structure definition?

Yes.  Fixed, thanks!

--D